Neutron sensitive tube



Dec 14, .1954 E. E. SHELDON NEUTRON SENSITIVE TUBE Original Filed April 16. 1947 INVENTOR; [om/g [MMUEL SHELDON. B X, I l

fl'r7o/2/vsv.

Unite States Patent Ofifice 2,697,181 Patented Dec. 14:, 1954 NEUTRON SENSITIVE TUBE Edward Emanuel Sheldon, New York, N. Y.

Original application April 16, 1947, Serial No. 741,803,

now Patent No. 2,555,423, dated June 5, 1951. Divided and this application June 1, 1951, Serial No. 229,492

6 Claims- (Cl. 313-61) This invention relates to an improved method and deto intensification of images produced by atomic particles such as neutrons;

One primary object of the present invention is to provide a device to produce intensified neutron images. This intensification will enable one to overcome the inefiiciency of the present fluoroscopic examinations. At the present level of illumination of the fluoroscopic image the human eye has to rely exclusively on scotopic (dark adaptation) vision, which is characterized by a tremendous loss of normal visual acuity in reference both to detail and to the contrast.

Another object of this invention is to make it possible to prolong the fluoroscopic examination since it will reduce markedly the strength of radiation affecting the patients body. Conversely, the exposure time or energy necessary for the radiography may be reduced.

Another object is to provide a method and device to produce sharper fluoroscopic and radiographic images than was possible until now.

The present intensifying devices concerned with reproduction of fluoroscopic images were completely unsatisfactory, as in the best of them amplification of the original image brightness of the order of 3 to 5 was achieved, while in order to obtain improvement in the visual acuity intensification of the brightness of the order of 1000 is obligatory. Without intensification of lumir nosity of at least the order of 1000 the eye is confined to so-called scotopic vision, at which it is not able to perceive definition and contrast of the fluoroscopic image. It is Well known that intensification of-the brightness of the fluoroscopic image can not be achieved by increase of energy of the X-ray or neutron radiation, as it will result in damage to the patients tissues. Therefore to obtain the objects of this invention a special neutron sensitive image tube had to be designed. This novel neutron image tube is characterized by elimination of the optical lens system present in other image tubes which resulted in 30 fold gain in the light reaching the photocathode. Then by the combined use of a novel photoemissive pick-up system and storage system of the tube, of a multiplier section of the tube, of a novel electron lmage amplifier system, of the electronic acceleration and of the electronic image diminution, the intensificatlon of the luminosity of the original image exceeding the ratio of l000-1 was accomplished. The elimination of the optical system present in other image tubes to focus the fluorescent image on the photocathode of the tube was accomplished by positioning within the neutron sensitive image tube of the screen, consisting of combination of neutron transparent, light reflecting layer, of neutron fluorescent or reactive layer, and

of the photoemissive layer. All layers are placed in close apposition to each other to prevent the loss of definition. Tho fluorescent and photoemissive layers are separated only by a very thin, light, transparent, chemically inactive, barrier layer. The previous combinations of fluorescent and photoemissive layers were not successful because of detrimental chemical interaction of both layers, due to lack of a barrier between them, therefore the introduction of light transparent barrier layer represents a very important part of this invention. The photoemissive layer of semi-transparent type is characterized by emission of electrons on the side opposite to the side of the incident light. The photo-electrons emitted from the photoemissive layer in a pattern corresponding to. the incident light pattern are focussed by *means of magnet c and/or electric fields on the target which serves as a secondary emission electrode and electron storage place. This division of the photo-surface into photoemissive section and secondary emission-storage section allows to obtain many-fold gain in efficiency as compared with the mosaic type of photoemissive surfaces where both photoemissive action and storage action are combined in one layer. In other cases in order to increase further the efficiency of this system the electron-emissive elfect and storage effect are separated from each other by using one special plate for electron emission and another one for electron storage. In some cases it may be desirable to use a cooling system for a photocathode and secondary emission electrode to inhibit thermionic emission.

Further intensification of the neutron image was obtained by the use of novel image amplification system.

The amplification section of the tube consists of one or a few screens each of them composed of a very thin, light-reflecting, electron pervious layer, of a fluorescent layer and of a photoemissive layer in close apposition to each other. It is necessary to include a very thin, light transparent, chemically inactive barrier layer between the fluorescent and photoemissive layers in order to prevent their chemical interaction. The electrons from the pickup section of the image tube are focussed by magnetic or electro-static fields on the fluorescent layer of a screen described above. The luminescence of the fluorescent layer of the amplification screen will cause the emission of electrons from the photoemissive layer of the screen. This process can be repeated a few times, using a few screens described above, resulting in 10-100 times intensification of the original electron image.

In another modification to be used in this invention there is an additional multiplier section which consists of a few stages of grid multiplier and can give an additional intensification of the electron image by secondary electron emission.

The electrons leaving the amplifying section are accelerated by means of high voltage electrostatic fields. The accelerating system can be of a conventional type well known in the art. Much better results with higher voltages will be achieved with an electrostatic multi-lens system.

Next the electron image is demagnified, which results in its additional intensification. The electron diminution of the image, in order to gain its intensification, is well known in the art, therefore does not have to be described in detail.

The diminished electron image is projected on the fluorescent screen at the end of the tube, where it can be viewed by the observer directly or by means of an optical magnifying eye piece through the light transparent end wall of the tube.

The use of an optical eye piece to magnify optically the electronically diminished image appearing on the fluorescent screen is also well known in the art, therefore does not need further description.

The combination of the above described features of the neutron sensitive image tube allows one to obtain intensification of the original neutron image, which was the primary objective of this invention. Having such a marked intensification of the original neutron image, it will be possible now to use a much finer grain of fluorescent screens than was practical until now and to improve this way detail and contrast of the final image, which was another purpose of this invention.

There are known in the art X-ray and neutron image tubes which have the feature of elimination of the optical focussing system. These tubes, however, rely for intensification completely on the electron acceleration. It is well known that the intensification of the electron image by acceleration can not be used beyond 20 kv. because of 'unavoidable ion emission from different parts of the tube, because .of-field emission .and .also..because .of .electron optical complications at the high voltage. The 20 kv. :acceleration results in only ten-fold intensification of :the original image, whereas for the purposes -:outlme d above thousand-fold intensification is necessary. It is obvious, therefore, that the use ofa novel p1ck-upsect1on .consisting of separate photoemissive and storage layers :and of novel cascade amplification system represents an invention without which the intensifying X-ray .orfneutron iimage tube could not operate. g

The invention will appear more clearly from -the following detailed description when taken in connection with the accompanying drawings by way of example only (of the preferred embodiments of the inventive idea.

In the drawings: 1 V

Figure 1 represents the X-ray image intensifying tube.

Figure 2 represents a modification of X-ray image intensifying tube. I p H ijFigure .3 represents a curved X-ray image intenslfying in e.

' Figure 4 represents neutron image intensifying tube.

Figure shows a modification of the .photoca'thode in the neutron image intensifying tube.

Figure .6 shows another modification of the photocathode in the neutron image intensifying tube.

'Fig. 1 shows an X-ray image intensifying tube. The X-ray image tube is not a part of the present invention. The description of this tube is necessary in order to explain later the operation of a neutron image intensifying tube, shown in Fig. 4.

The face .1 of the image tube shown in Fig. 1 must be of a material transparent to the type of radiation to be used. Inside theface of the tube there is a very thin, light reflecting aluminum layer 2 which prevents the loss of light from the fluorescent screen 3. An extremely thin barrier layer 4 separates the fluorescent screen-3 from the photoemissive layer 5. The fluorescent 3 and photoemissive layers 5 should be correlated so that under the influence of the particular radiation used there is obtained a maximum output of photoemission. More particularly the fluorescent screen should be composed of a material having its greatest sensitivity to the type of radiation to be used, and the photoemissive material likewise should have its maximum sensitivity 'to the wave length,emitted by the fluorescent screen. Fluorescent substances that may be used are willemite, or other zinc silicates, zinc se'lenides, zinc sulphides, calcium fluoride or calcium tungstate, with or without activators. The satisfactory photoemissive materials will be caesium oxide, caesium oxide activated by silver, caesium with antimony, with bismuth or arsenic or antimony, with lithium or potassium. The barrier layer 4 between the fluorescent and photoemissive surfaces can be an exceedingly thin transparent film of mica, glass, of organic substance such as nitrocellulose or gelatine, of silicon or of a suitable plastic. The thickness of separating layer should not exceed 0.15 millimeter.

The electron image obtained on the photoemissive layer 5 is not projected on the secondary emission target Sal-5b by means of magnetic and/or electrostaticfield.

The electron image obtained in the pick-up section is then transferred to the first screen of the amplifying section 7 by means of focussing magnetic or electrostatic field, which is not indicated since it is well known in the art and would only serve to complicate the illustration.

The amplifying section 7 uses one or a few successively arranged special screens, Fig. 2, each of them consisting of an electron pervious, light-reflecting layer 8, of a fluorescent layer 9, of a light transparent barrier layer 10 and of a photoemissive layer 11. Fluorescent substances that may be used are willemite or other zinc silicates, zinc selenides, zinc sulphide, calcium fluoride or calcium tungstate with or without activators. The satisfactory photoemissive materials will be caesium oxide, caesium oxide activated by silver, caesium with antimony, with bismuth or arsenic or antimony with lithium or potassium. The barrier layer 10 between the fluorescent and photoemissive surfaces can be an exceedingly thin transparent film of mica, glass, or organic substance such as. e. g. nitrocellulose or gelatine, of silicon, or of a-suitable plastic. The amplification achieved by this systern results in marked intensification of the original image.

In some applications it may be preferable to use in additional elements.

conjunction with amplifying system the electron multiplier section 6 consisting of one or a few stages of secondary electron multipliers vwhich serves to intensify further the electronic image. In such a case the electron image from the pick-up section of the tube is focussed by means of magnetic field on the first stage of the multiplier section. The secondary electrons from the first stage are focussed the .same way on the second stage of the multipliersection and 'so "on.

The 'electronsemerging fromthe amplifying section are now accelerated and image by means of electromagnetic fields 19 to the "desired velocity, giving this further intensification of the electron image. Next the electron image is diminished by means of electro-magnetic lenses 19a to the desired size, resulting in image intensification proportional to the square power of the linear diminution and is projected through the electron pervious, light reflecting aluminum layer 20 on the fluorescent screen 21 made of finegrains of Zn0,.Zn silicates or ZnS with appropriate activators where it can be viewed by the observer. .In some cases it may be more d'esirable to have the fluorescent screen mounted outside the vacuum tube; insuch cases thin "electron transparent layer of chromium or aluminum is placed on the end wall 22 of the vacuum tube made of fernico glass. The image appearing on the fluorescent screen can be viewed directly or by means of an optical eye-piece 23 giving the desired optical magnification of the image. In other cases the fluorescent screen 21 is substituted by .photographic layer or by photographic layer in combination with fluorescent screen, thus permitting one toobtain a permanent record of the electron image.

In another alternative of this invention the tube is curved, Fig. 3, and theelectron beam is deflected by proper magnetic fields. This arrangement will prevent ghe positive ions from reaching the photoemissive sec- 101'1. p The embodiment of the invention adapted for intensification of neutron images is illustrated in Fig. 4, wherein a neutron reactive layer 26 preferably from the group boron, l1thium,-gado1inium and uranium or of .pa'rafiine is placed on the face of the image tube to act as a photocathode. The protons or electrons liberated from this layer 26 under the impact of neutron radiation will strike through a thin electron .pervious chemically inactive barr er layer 40, a suitable fluorescent layer 27, causing it to fluoresce and activate a suitable photoemissive layer 29 through the light transparent barrier layer 28. In other cases a neutron reactive layer of cadmium or copper will be more advantageous, because of its gamma emls'sion.

In some cases it may be more desirable, Fig. 5, to eliminate the fluorescent layer 27 and to cause protons and electrons from the layer 26 to act on electron-emissive layer 31 e ther by apposition, in which case electron pervious chemically inactive barrier layer 30 has to be used to prevent chemical interaction of adjacent layers, or by focussing them on electron-emissive layer 31 with magnetic or electrostatic fields, Fig. 6.

The fluorescent layer to be used in the neutron sensitive tube may be of similar composition as described above in the X-ray sensitive image tube, but it has also to be adapted to respond most efiiciently to the radiation emitted from neutron sensitive layer by enriching it with proper The photoemissive layer has again to be correlated with spectral emission of fluorescent layer. The amplifying system, the multiplier system, the accelerating system, the electronic diminution are the same for neutron sensitive image tube and for X-ray sensitive image tube.

Although the particular embodiments and forms of this invention have been illustrated and, it is understood that modifications may be made by those skilled in the art, without departing from the full scope and spirit of the foregoing disclosure.

Having thus described my invention, I claim as new and desire to secure by Letters Patent:

1. In atomic-particles-sensitive tubes, an evacuated envelope, having in combination, a composite photocathode consisting of a fluorescent layer having distributed throughout said layer elements reactive to atomic particles, a light transparent separating layer, and of a photoelectric layer, said light transparent layer of a thickness not exceeding 0.15 millimeter, accelerating means and an electron reactive screen spaced apart from said photocathode.

2. In a device as defined in claim 1, said composite photocathode having an atomic particles reactive layer, a light opaque layer, a fluorescent layer reactive to radiation emitted by said atomic particles, reactive layer, a separating layer of a thickness not exceeding 0.15 millimeter and independent of walls of said tube, and a photoemissive layer.

3. In a device as defined in claim 1, said photocathode having an atomic particles reactive layer, a fluorescent layer reactive to radiation emitted by said atomic particles reactive layer, a light transparent layer of a thickness not millimeter, and a photoemissive layer.

spaced apart from said photocathode.

5. In atomic particles sensitive image tubes, an evacuted envelope having in combination atomic particles reactive means for receiving an atomic particles image, fluorescent means responsive to radiation emitted by said atomic particle reactive means, light transparent separating means References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,158,853 Coolidge May 16, 1939 2,344,042 Kallmann et a1 Mar 14, 1944 2,344,043 Kallmann et al Mar. 14, 1944 2,506,018 Flory et al May 2, 1950 2,525,832 Sheldon Oct. 17, 1950 2,555,423 Sheldon June 5, 1951 

1. IN ATOMIC-PARTICLES-SENSITIVE TUBES, AN EVACUATED ENVELOPE, HAVING IN COMBINATION, A COMPOSITE PHOTOCATHODE CONSISTING OF A FLUORESCENT LAYER HAVING DISTRIBUTED THROUGHOUT SAID LAYER ELEMENTS REACTIVE TO ATOMIC PARTICLES, A LIGHT TRANSPARENT SEPARATING LAYER, AND OF A PHOTOELECTRIC LAYER, SAID LIGHT TRANSPARENT LAYER OF A THICKNESS NOT EXCEEDING 0.15 MILLIMETER, ACCELERATING MEANS AND AN ELECTRON REACTIVE SCREEN SPACED APART FROM SAID PHOTOCATHODE. 